Quadrature amplitude modulation for time-reversal systems

ABSTRACT

Time-reversal wireless communication includes: at a base station, receiving a probe signal from a terminal device; generating a signature waveform that is based on a time-reversed signal of a channel response signal derived from the probe signal; performing quadrature amplitude modulation (QAM) on a transmit signal to generate a quadrature amplitude modulated signal; and generating a transmission signal based on the quadrature amplitude modulated signal and the signature waveform.

TECHNICAL FIELD

This disclosure generally relates to quadrature amplitude modulation fortime-reversal wireless systems.

BACKGROUND

A time-reversal division multiple access (TRDMA) system provides acost-effective single-carrier technology for broadband communicationsand at the same time leverages the degrees of freedom in a large numberof multi-paths to form a unique high-resolution spatial focusing effect.In some time-reversal communication systems, when a transceiver Aintends to transmit information to a transceiver B, transceiver B firstsends a delta-like pilot pulse that propagates through a scattering andmulti-path environment, and the signals are received by transceiver A.Transceiver A transmits time-reversed signals back through the samechannel to transceiver B. Based on channel reciprocity, a time-reversalcommunication system leverages the multi-path channel as a matchedfilter, i.e., treats the environment as a facilitating matched filtercomputing machine, and focuses the wave at the receiver in both spaceand time domains.

SUMMARY

In general, in one aspect, a method for time-reversal wirelesscommunication includes at a base station, receiving a probe signal froma terminal device; generating a signature waveform that is based on achannel response signal derived from the probe signal; performingquadrature amplitude modulation (QAM) on a transmit signal to generate aquadrature amplitude modulated signal; and generating a transmissionsignal based on the quadrature amplitude modulated signal and thesignature waveform.

Implementations of the method may include one or more of the followingfeatures. Performing quadrature amplitude modulation on the transmitsignal can include dividing the transmit signal into a first part and asecond part, applying amplitude modulation on the first part to generatean in-phase part of the quadrature amplitude modulated signal, andapplying amplitude modulation on the second part to generate aquadrature part of the quadrature amplitude modulated signal. Thesignature waveform can include a complex signal having a real part andan imaginary part. Generating a signature waveform can includegenerating a signature waveform based on a time-reversed signal of thechannel response signal. Generating the transmission signal can includeperforming a convolution of the signature waveform and the quadratureamplitude modulated signal or a modified version of the quadratureamplitude modulated signal. The modified version of the quadratureamplitude modulated signal can include an up-sampled version of thequadrature amplitude modulated signal, and generating the transmissionsignal can include performing a convolution of the signature waveformand the up-sampled version of the quadrature amplitude modulated signal.The transmit signal can include a digital transmit signal. Performingthe quadrature amplitude modulation can include encoding data bits ofthe transmit signal based on Gray codes, and mapping

Gray-coded data bits to quadrature amplitude modulated symbols.Generating the signature waveform can include generating a signaturewaveform that is a time-reversed conjugate signal of the channelresponse signal. Performing quadrature amplitude modulation on atransmit signal can include performing 4 QAM, 16 QAM, 64 QAM, or 256QAM.

In general, in another aspect, a method for time-reversal wirelesscommunication includes performing quadrature amplitude modulation on afirst transmit signal to generate a first quadrature amplitude modulatedsignal; performing quadrature amplitude modulation on a second transmitsignal to generate a second quadrature amplitude modulated signal;generating a first transmission signal based on the first quadratureamplitude modulated signal and a first signature waveform associatedwith a first terminal device; generating a second transmission signalbased on the second quadrature amplitude modulated signal and a secondsignature waveform associated with a second terminal device; andgenerating a combined transmission signal by adding a real part of thefirst transmission signal with a real part of the second transmissionsignal to generate an in-phase part of the combined transmission signal,and adding an imaginary part of the first transmission signal with animaginary part of the second transmission signal to generate aquadrature part of the combined transmission signal.

Implementations of the method may include one or more of the followingfeatures. The method can include receiving a first probe signal from thefirst terminal device; and generating the first signature waveform basedon a time-reversed signal of a first channel response signal derivedfrom the first probe signal. Generating the first signature waveform caninclude generating a first signature waveform that is a time-reversedconjugate signal of the first channel response signal. Performingquadrature amplitude modulation on the first transmit signal can includedividing the first transmit signal into a first part and a second part,applying amplitude modulation on the first part to generate an in-phasepart of the first quadrature amplitude modulated signal, and applyingamplitude modulation on the second part to generate a quadrature part ofthe first quadrature amplitude modulated signal. The first signaturewaveform can include a complex signal having a real part and animaginary part. Generating the first transmission signal can includeperforming a convolution of the first signature waveform and the firstquadrature amplitude modulated signal or a modified version of the firstquadrature amplitude modulated signal. The modified version of the firstquadrature amplitude modulated signal can include an up-sampled versionof the first quadrature amplitude modulated signal, and generating thefirst transmission signal can include performing a convolution of thefirst signature waveform and the up-sampled version of the firstquadrature amplitude modulated signal. The transmit signal can include adigital transmit signal. Performing the quadrature amplitude modulationcan include encoding data bits of the transmit signal based on Graycodes, and mapping Gray-coded data bits to quadrature amplitudemodulated symbols. Performing quadrature amplitude modulation on atransmit signal can include performing 4-QAM, 16-QAM, 64-QAM, or256-QAM.

In general, in another aspect, a system for time-reversal wirelesscommunication includes a first device that includes an input circuitconfigured to receive a probe signal transmitted wirelessly from asecond device through multiple propagation paths, and a data processor.The data processor is configured to generate a signature waveform thatis based on a channel response signal derived from the probe signal;perform quadrature amplitude modulation (QAM) on a transmit signal togenerate a quadrature amplitude modulated signal; and generate atransmission signal based on the quadrature amplitude modulated signaland the signature waveform.

Implementations of the system may include one or more of the followingfeatures. The transmit signal can be divided into a first part and asecond part, amplitude modulation can be applied on the first part togenerate an in-phase part of the quadrature amplitude modulated signal,and amplitude modulation can be applied on the second part to generate aquadrature part of the quadrature amplitude modulated signal. Thesignature waveform can include a complex signal having a real part andan imaginary part. The data processor can be configured to generate thesignature waveform based on a time-reversed signal of the channelresponse signal. A convolution of the signature waveform and thequadrature amplitude modulated signal or a modified version of thequadrature amplitude modulated signal can be performed. The modifiedversion of the quadrature amplitude modulated signal can include anup-sampled version of the quadrature amplitude modulated signal, and aconvolution of the signature waveform and the up-sampled version of thequadrature amplitude modulated signal can be performed. The transmitsignal can include a digital transmit signal. Data bits of the transmitsignal can be encoded based on Gray codes, and Gray-coded data bits canbe mapped to quadrature amplitude modulated symbols. A signaturewaveform that is a time-reversed conjugate signal of the channelresponse signal can be generated. 4 QAM, 16 QAM, 64 QAM, or 256 QAM canbe performed on the transmit signal.

The methods and systems described above can be implemented in variousdevices and systems, e.g., mobile phones, vehicles (e.g., cars, ships,airplanes), robots, unmanned aerial vehicles, thermostats,refrigerators, lighting control systems, personal desktop computers,laptop computers, tablet computers, network routers, and televisions.

The details of one or more implementations of time-reversal wirelesscommunication systems are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an exemplary environment for operating atime-reversal system.

FIG. 2A is a graph of an exemplary recorded channel impulse responsewaveform.

FIG. 2B is a graph of an exemplary time-reversed waveform generated byreversing the waveform of FIG. 2A with respect to time.

FIG. 3 is a diagram showing an exemplary environment for operating atime-reversal system having multiple receivers.

FIG. 4 is a diagram of an exemplary multi-user time reversalcommunication system.

FIG. 5 is a diagram of various phases of time-reversal wirelesscommunication.

FIG. 6 is a block diagram of a transmitter in a quadrature amplitudemodulation (QAM) time-reversal system.

FIG. 7 is a diagram of an implementation of a transmitter in aquadrature amplitude modulation time-reversal system.

FIG. 8 is a block diagram of a transmitter that generates downlinksignals for multiple devices.

FIG. 9 is a diagram of bit/symbol mapping for 4 QAM or QPSK (quadraturephase shift keying).

FIG. 10 is a diagram of bit/symbol mapping for 16 QAM.

FIG. 11 is a diagram of a receiver in a quadrature amplitude modulationtime-reversal system.

FIG. 12 show graphs of channel impulse response signals.

FIG. 13 show graphs of modulated waveforms using QPSK (4 QAM).

FIG. 14 is a graph showing a comparison of received signals of variousquadrature amplitude modulation symbols transmitted when using (QPSK) 4QAM.

FIG. 15 is a graph showing a comparison of the bit error rateperformance for a fixed bit rate.

FIG. 16 is a graph showing a comparison of the bit error rateperformance for a fixed rate back-off factor.

FIGS. 17 and 18 are flow diagrams of processes for time-reversalwireless communication.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

This disclosure describes a novel time-reversal wireless communicationsystem that uses quadrature amplitude modulation. In time-reversalcommunication systems, information bits are modulated into symbols, andthe symbols are then modulated by transmit waveforms for transmission.By utilizing channel reciprocity, the time-reversal waveforms cancollect much of the energy in the multipath channel, resulting in atemporally spiky signal power focused at the intended receiver.Quadrature amplitude modulation (QAM) maps two digital bit-streams intoan analog complex symbol-stream for transmission. Each bit-stream ismodulated by amplitude modulation, and the two amplitude-modulatedsymbol-streams are then combined into one complex symbol-stream. Theinformation of the digital bits is carried by both the amplitude and thephase of the complex symbol-stream. In the following, we describe atransceiver architecture incorporating quadrature amplitude modulationin time-reversal systems with arbitrary transmit waveforms.

In this description, depending on context, the term “user” may refer toa device. For example, in a system that has multiple devicescommunicating with a base station, the term “multi-user uplink” refersto the uplink by multiple devices, and the term “inter-userinterference” refers to the interference among various devices. Thetime-reversal division multiple access technology has a wide variety ofapplications. For example, an intelligent house may include one or morecomputers that communicate wirelessly with several sensors (e.g.,temperature, humidity, light, and motion sensors), meters (e.g.,electricity and water meters), appliances (e.g., refrigerator, oven,washing machine), electronic devices (e.g., television, digital videorecorder, audio/video system, telephone, digital photo album,intelligent lamp, security system), climate control systems (e.g., fans,thermostats for air conditioning and heating, motorized shades), powergenerators (e.g., backyard wind turbine, solar panel, geothermal energysystem). A home lighting system may include a controller that controlsintelligent light bulbs (e.g., bulbs that use light emitting diodes(LED) or laser technology) to adjust color and/or brightness of thebulbs. The controller may communicate with the intelligent bulbs usingtime-reversal wireless communication. The communication module at thelight bulb using time-reversal technology can be made at a lower costcompared to, e.g., a communication module using Wi-Fi or Bluetoothtechnology. Thermostat controllers, smoke detectors, security systems,and phone systems may communicate with one another using time-reversalcommunication technology. For example, a smoke/carbon monoxide detectorthat detects smoke or carbon monoxide may communicate with thethermostat to shut off the boiler, communicate with a home securitysystem that notifies the fire department, or communicate with a phonesystem that sends an alert text or voice message or e-mail to the homeowner. The smoke detector may have motion sensors that can detect thepresence of people, and may announce voice messages in case ofemergency. For example, if a home has smoke detectors installed atseveral locations, a first smoke detector that detects smoke in thefirst floor kitchen may communicate with a second smoke detector locatedon the third floor bedroom where a home owner is sleeping to cause thesecond smoke detector to broadcast an announcement alerting the homeowner that smoke is detected in the first floor kitchen.

A home or office may have multiple devices that have clocks. The devicesmay communicate with a controller that provides an accurate time signalso that all the devices in the home or office can be synchronized. Thecontroller can provide the time signals to the various devices usingtime-reversal wireless communication.

Plant care (e.g., in a greenhouse, home, or office building) may bepartially automated by use of sensors that sense soil conditions andprovide information to indicate whether watering or fertilization isneeded, the amount of water and fertilizer required, and the type offertilizer needed. For example, a plant care module may be inserted intothe soil or planting medium adjacent to each plant, in which the plantcare module may have a storage that stores information about the plant,such as the type of plant, the level of moisture that should bemaintained, and how often and what type of fertilizers need to beapplied, the date and time in which water or fertilizer was applied, andthe type and amount of fertilizer applied. The plant care module mayhave sensors that sense the soil conditions and a communication moduleto communicate with an automated plant care system, such as a robot thatcan provide water and fertilizers based on the information sent from theplant care module. The plant care module can communicate with a lightingsystem that controls the amount of light provided to the plant and atemperature control system that controls the temperature in the vicinityof the plant. For example, the communication module can usetime-reversal wireless communication technology that requires littlepower, and the plant care modules can be powered by solar cells toeliminate the need to change batteries.

Vehicles may have time-reversal wireless communication modules thatcommunicate with various sensors, beacons, or data processing modules ingarages, driveways, or buildings. For example, garages may beretrofitted with sensors and beacons to assist vehicles (e.g.,autonomous vehicles) to find and park into parking spaces. Robots mayhave time-reversal wireless communication modules that communicate withvarious sensors, beacons, or data processing modules in homes or officebuildings to assist in navigation or to provide information about theenvironmental conditions or other information (e.g., tasks that need tobe performed at particular locations). As discussed below, time-reversalwireless communication has an asymmetrical nature in which the basestation performs most of the signal processing as both a transmitter(for the downlink) and receiver (for the uplink), allowing the use oflow complexity terminal devices. Thus, a large number of terminaldevices (e.g., sensors) can be deployed at a low cost. Because theterminal devices require little power, they can be powered by solarcells or piezoelectric components to eliminate the need to rechargebatteries, or be power by batteries that last for a long lifetime.

In an assembly plant, critical components may have embedded processorsthat store and process data and communicate with one or more externalcontrollers using time-reversal wireless communication technology toensure that the critical components function properly, and have beenproperly processed in previous stages before moving to the next stage.For example, an engine of a vehicle can have an embedded module having adata processor and storage that stores information about all the teststhat have been performed in previous stages before being assembled withthe car chassis. An airplane may be assembled from componentsmanufactured by companies in different countries. Each component canhave an embedded module that has a data processor and storage thatstores information about the component, such as characteristics of thecomponents and results of tests that have been performed on thecomponent. The embedded modules may communicate with one or moreexternal controllers using time-reversal wireless communicationtechnology. Use of such embedded modules can increase the quality of thefinal products by ensuring that each component has been properlyprocessed and tested.

In some examples, a controller may communicate with multiple sensors ordevices using time-reversal wireless communication, and communicate withother devices using other communication protocols. For example, acontroller may communicate with intelligent light bulbs, temperaturesensors, and power meters using time-reversal wireless communication,and communicate with a smart phone using Wi-Fi communication. Thecontroller serves as a bridge between devices that use low costtime-reversal communication modules and devices (e.g., smart phones ornetwork routers) that follow Wi-Fi or Bluetooth communication protocols.

For example, an intelligent factory may include one or more computersthat communicate wirelessly with robots working in assembly lines,vehicles that move supplies and assembled products, climate controlsystems, security systems, inventory control systems, and power systems.For example, a laboratory may include one or more computers thatcommunicate wirelessly with instruments that monitor parameters whenconducting experiments.

In the examples above, the computer (or controller) can communicate withthe devices using time-reversal division multiple access technology thatuses the environment to provide spatial filtering, allowing a largenumber of devices to communicate with the computers simultaneously.Compared to using previous wireless communication technologies, such asWi-Fi or Bluetooth, time-reversal division multiple access has theadvantage that the additional cost for enabling each device tocommunicate with the computer (or controller) is small because thedevice itself does not need to perform complicated signal processing.Most of the signal processing is performed at the computer (orcontroller). The power consumption by each device for enabling wirelesscommunication is also much smaller compared to previous wirelesstechnologies.

For example, a swarm of robots and/or unmanned aerial vehicles (drones)can communicate wirelessly with each other using time-reversal divisionmultiple access technology. For example, some of the robots and/ordrones can function as controllers that perform more complicated signalprocessing, while the other robots and/or drones function as terminaldevices that do not need to perform the complicated signal processing,For rescue robots working in disaster situations, such as in partiallycollapsed buildings or underground tunnels, multi-path interference maybe especially severe for conventional wireless technologies based on,e.g., Wi-Fi or Bluetooth. The multiple signal paths can be usedadvantageously by time-reversal division multiple access technology, inwhich the environment provides spatial filtering. For small aerialdrones that have small batteries or use solar power, reducing the energyrequired for wireless communication will allow the energy to be usedelsewhere, such as increasing the flight time of the drones.

For example, wearable devices can communicate wirelessly with each otherand/or with a controller using time-reversal division multiple accesstechnology. For example, wearable energy generating devices (such aspiezoelectric power generators) can be integrated into clothing and/orshoes, and used to provide power to sensors that monitor bodyparameters. For example, the sensors can be used to measure respiratorypatterns, heart beat patterns, walking/running patterns, and/or sleepingpatterns. The sensors can wirelessly send measured data to a controller(e.g., smart phone or computer) that processes the collected data. Theprocessed data can be presented in a user-friendly graphical interface.Because time-reversal division multiple access technology requires verylittle power, the wearable sensors can be powered by the energygenerated by the movements of the body (e.g., using piezoelectriccomponents). In some examples, the wearable energy generating devicescharge a rechargeable battery, which in turn powers the sensors. In thatcase, use of time-reversal division multiple access technology removesthe requirement to recharge the battery through external power sources,or increases the time duration between recharging. Because time-reversaldivision multiple access technology requires less power than othercommunication technologies such as Wi-Fi or Bluetooth, there may be lessside effects on the human body due to exposure to electromagneticsignals.

The time-reversal division multiple access scheme uses the multi-pathchannel profile associated with each user's location as alocation-specific signature for the user.

Each path of the multi-path channel is treated as a virtual antenna inthe time-reversal division multiple access system, which collectivelyresults in very high-resolution spatial focusing with “pin-point”accuracy. The computer (or controller) may function as a base station orbe coupled to a base station that performs most of the signal processingwhen transmitting signals to the devices and receiving signals sent fromthe devices.

Because the signals are transmitted through multiple propagation pathshaving various propagation lengths, a transmitter and a receiver need toobtain information about the overall system, e.g., information about thecommunication channel and information about time synchronization. Insome implementations, such information is obtained in a channel probing(CP) phase and a data transmission (DT) phase of a time-reversalcommunication system. In the channel probing phase, the transmitteracquires channel information to realize the focusing effects, while inthe data transmission phase, the receiver acquires timing information tosynchronize and sample relevant signals. The process of obtainingchannel information in the channel probing phase and obtainingsynchronization in the data transmission phase is referred to astime-reversal handshaking The following describes several handshakingmethods for obtaining necessary channel and timing information forenabling time-reversal communication systems.

Overview of Time-Reversal System

The following provides an overview of a time-reversal system. Referringto FIG. 1, a time-reversal system can be used in an environment havingstructures or objects that may cause one or more reflections of wirelesssignals. For example, a venue 102 may have a first room 104 and a secondroom 106. When a first device 108 in the first room 104 transmits asignal to a second device 110 in the second room 106, the signal canpropagate in several directions and reach the second device 110 bytraveling through several propagation paths, e.g., 112, 114, and 116.The signal traveling through multiple propagation paths is referred toas a multipath signal. As the signal travel through the propagationpaths, the signal may become distorted and noise may be added. Themultipath signal received by the second device 110 can be quitedifferent from the signal transmitted by the first device 108.

For example, referring to FIG. 2A, when the first device 108 sends apulse signal, the signal received by the second device 110 may have awaveform 120. The waveform 120 is referred to as the channel impulseresponse signal. Referring to FIG. 2B, a time-reversed waveform 130 canbe generated by reversing the waveform 120 with respect to time. If thesecond device 110 sends a signal having the waveform 130, the signalwill propagation in various directions, including through propagationpaths 112, 114, and 116 (in reverse direction relative to thepropagation direction of the impulse signal), and reach the first device108. The multipath signal received at the first device 108 forms animpulse signal that is similar to the impulse signal previously sentfrom the first device 108 to the second device 110.

The waveforms 120 and 130 shown in FIGS. 2A and 2B are merely examples.The waveforms in time-reversal systems can vary depending on, e.g., theenvironment and the information or data being transmitted.

When the second device 110 intends to transmit a data stream to thefirst device 108, the second device 110 uses normalized time-reversedconjugate signals as a basic waveform. The second device 110 loads thedata stream on the basic waveform, and transmits the signal through thewireless channel. Usually the sampling rate is higher than the baudrate. The signal received at the receiver is the convolution of thetransmitted signal and the channel impulse response, plus additive whiteGaussian noise.

The first device 108 performs a simple adjustment to the received signaland down-samples it to recover the data stream transmitted by the seconddevice 110.

In some examples a transmitter may send signals to two or more receiversat the same time. The transmitted signal travel through multiplepropagation paths to each receiver. Because the receivers are positionedat different locations, the multipath signals travel through differentpropagation paths to reach the receivers, different multipath signalswill be received at different receivers. By carefully constructing thewaveform of the signal sent from the transmitter, it is possible toallow each receiver to receive data intended for the receiver withsufficiently high quality.

Referring to FIG. 3, the first device 108 may communicate with thesecond device 110 and a third device 140. During a hand-shaking process,the second device 110 sends a probe signal that travels throughpropagation paths 112, 114, and 116 to the first device 108. The probesignal can be, e.g., a pulse signal, a signal that has a predeterminedwaveform, or a signal that includes symbols. The first device 108records the received waveform, and determines channel and timinginformation for the first multipath channel. The third device 110 sendsa probe signal that travels through propagation paths 142, 144, and 146to the first device 108. The first device 108 records the receivedwaveform, and determines channel and timing information for the secondmultipath channel.

The first device 108 constructs a downlink signal that includes a firstportion that is intended to be received by the second device 110 afterthe downlink signal propagates through the first multipath channel(including propagation paths 112, 114, and 116). The first portion isdetermined based on a first time-reversed multipath channel response,and is designed such that, after propagating through the first multipathchannel, the signal received by the second device 110 includes firsttiming information and a first payload. The first timing informationenables the second device 110 to accurately determine the beginning ofthe first payload, which includes data intended for the second device110.

The downlink signal includes a second portion that is intended to bereceived by the third device 140 after the downlink signal propagatesthrough the second multipath channel (including propagation paths 142,144, and 146). The second portion is determined based on a secondtime-reversed multipath channel response, and is designed such that,after propagating through the second multipath channel, the signalreceived by the third device 140 includes second timing information anda second payload. The second timing information enables the third device140 to accurately determine the beginning of the second payload, whichincludes data intended for the third device 140.

In the example of FIG. 3, the first device 108 may use either anomnidirectional antenna or a directional antenna for broadcasting thedownlink signal, as long as the downlink signal reaches each of thesecond and third devices 110 and 140 through multiple propagation paths.

In some examples, a multiple input multiple output (MIMO) system may beused in which the device operating as a transmitter has multipletransmit antennas, and each of the devices operating as a receiver hasmultiple receive antennas.

System Architecture

A time-reversal division multiple access architecture has two parts, thedownlink part and the uplink part. In a time-reversal division multipleaccess downlink system, a base station (BS) transmits multiple datastreams to several users simultaneously, in which each user isassociated with a unique multi-path profile in rich-scatteringenvironments. The time-reversal division multiple access downlink schemeexploits the spatial degrees of freedom of the environment and focusesthe useful signal power mostly at the intended locations. Time reversalmirrors (TRMs) at the base station first time-reverse the channelresponse of each user's channel as the user's signature waveform, andthen embed these signatures into the corresponding data streams. Thetransmitted signal from the base station in the time-reversal divisionmultiple access downlink is a mixed signal that includes the dataintended to be sent to several users (including different data intendedfor different users). When the combined signal propagates to a certainuser through the corresponding multipath channel, a large number ofmulti-paths having identical phases will automatically resonate at thisuser's location, resulting in the spatial focusing of the power of theuseful signal component that carries this user's data.

Within the time-reversal division multiple access framework, moresophisticated signature waveforms than the basic time-reversal waveformcan be derived based on the multi-path channel responses to furtherimprove the performance of the time-reversal division multiple accessdownlink system, when additional computational complexity is affordableat the base station. One desirable feature of the time-reversal divisionmultiple access downlink scheme is that most of the complexity incommunication can be shifted to the base station side, facilitating lowcomplexity in communication components at the end-users.

A time-reversal division multiple access uplink scheme can beimplemented in which the terminal devices have low complexitycommunication components. The time-reversal division multiple accessuplink scheme shares a strong duality in the mathematical structure withthe downlink without increasing the complexity of the end-users. Avirtual spatial focusing effect can be observed in the user's signaturedomain at the base station. Similar to that of the downlink scheme, thevirtual spatial focusing effect enables the base station to use theuser's time-reversal signature waveform to extract the useful componentfrom the combined received signals, allowing multiple users accessingthe base station simultaneously. Additionally, unlike many otherconventional communications paradigms that adopt symmetricarchitectures, the uplink scheme shares the same processing power andchannel knowledge at the base station with the downlink, allowing theend-user's communication component to have a low complexity.

Referring to FIG. 4, an exemplary multi-user time reversal communicationsystem 150 includes a base station (BS) 152 and multiple terminaldevices (e.g., 154 a, 154 b, 154 c, collectively 154). Each of theterminal devices 154 is associated with a multi-path wireless channel(e.g., 156 a, 156 b, 156 c, collectively 156) between itself and thebase station 152. Each multi-path wireless channel 156 in the figurerepresents two or more multiple signal propagation paths between thecorresponding terminal and the base station. In some implementations,all the devices (including the base station 152 and the terminals 154)can operate at the same frequency band, and the system operates inmulti-path environments. For the downlink, the base station 152 can sendmultiple messages (either independent or non-independent) simultaneouslyto multiple selected terminals 154. For the uplink, multiple terminals154 can send their own messages to the base station 152 simultaneously.

After each terminal device 154 registers with the base station 152,hand-shaking occurs periodically between the base station 152 and eachof the registered terminal devices 154 so that the base station 152 canmaintain up-to-date records of the channel responses associated with theterminal devices 154. The hand-shaking also enables the base station 152and the terminal devices 154 to obtain accurate timing information.

In the system 150, the time-reversed waveform is able to boost thesignal-to-noise ratio at the receiver with a low transmitter complexity.The receiver complexity is also low due to the one-tap detection, thatis, the receiver detects the received signal using one sample instead ofmore complicated receiver equalization.

Referring to FIG. 5, in general, in a time reversal communicationsystem, communication between a first device and a second device occursin two phases: the channel probing phase 160 and the data transmissionphase 162. In the diagram of FIG. 5, the horizontal axis representstime. In the description below, when two devices are performing timereversal wireless communication, the device that sends payload data isreferred to as the “transmitter” and the device that receives payloaddata is referred to as the “receiver,” even though during the channelprobing phase the “receiver” may send a channel probing signal to the“transmitter.”

In the channel probing phase 160, the receiver first sends a channelprobing signal 164 that propagates through a scattering and multi-pathenvironment, and the signals are received by the transmitter. Thetransmitter estimates the channel response (CR). In the datatransmission phase 162, the transmitter transmits the time-reversedchannel response with useful information back through the same channelto the receiver. Due to channel reciprocity, the wave is focused at thereceiver at a particular time instant. The receiver simply samples atthe focusing time instant to obtain the useful information.

During the channel probing phase, the transmitter needs to obtainchannel information that is sufficiently accurate in order to realizethe focusing effects. In the data transmission phase, the receiver needsto obtain sufficiently accurate timing information in order tosynchronize and sample the signal at the correct time. The process ofobtaining channel information in the channel probing phase and gettingsynchronized in the data transmission phase is referred to as timereversal handshaking Techniques for time-reversal handshaking isdescribed in U.S. patent application Ser. No. 14/183,648, titled“Handshaking Protocol For Time-Reversal System,” filed on Feb. 19, 2014,the contents of which are incorporated by reference in their entirety.

The time reversal communication system 150 has many advantages. Thesystem 150 leverages the multi-path channel as a matched filter, i.e.,harvesting energy from the scattering environment, it is highly energyefficient. Moreover, because the receiver only needs to sample at theparticular time instant without sophisticated processing to obtain theuseful information, the receiver is easy to implement, i.e., thecomputational complexity at the receiver can be low. The advantagesmentioned above depend on having adequate knowledge about the overallsystem at both the transmitter and the receiver, e.g., the channelinformation and time information. Such information needs to be obtainedin the channel probing phase and the data transmission phase of the timereversal communication system.

Referring to FIG. 6, in some implementations, a transmitter 170 in thesystem 150 processes information bits 172 and generates a downlinksignal 174 for transmission to a receiver. For example, the transmitter170 can be part of the base station 152, and the receiver can be part ofthe terminal device 154. The processing of information bits fortransmission in the time reversal system 150 includes two phases. In thefirst phase, the receiver first sends a probe signal, which is receivedby the transmitter as a channel response signal. The probe signal caninclude arbitrary channel training signals. The transmitter 170generates a signature waveform based on the channel response signal. Inthe second phase, a quadrature amplitude modulation module 176 maps theinformation bits 172 into a stream of complex symbols 178. Anup-sampling module 180 up-samples the complex symbols 178 to generate anup-sampled symbol stream 182. A convolution module 184 performs aconvolution of the symbol stream 182 and the signature waveform (whichcan be a complex waveform calculated based on the channel response) togenerate a signal 186. The signal 186 is passed through adigital-to-analog converter 188 that generates an analog signal 190. Acarrier modulation module 192 modules the analog signal 190 with acarrier signal having a carrier frequency f_(c) to generate the carriermodulated signal 174, which is transmitted through an antenna.

Each block in FIG. 6 can be implemented in various ways. For example,FIG. 7 is a diagram of a transmitter 200 having components that processthe in-phase and quadrature components of the symbol stream and thein-phase and quadrature components of the signature waveform.

The transmitter 200 modulates information bits 202 into complex symbolsdenoted by X[m] using quadrature amplitude modulation, where m is thetime index. The quadrature amplitude modulation can be performedparallelly in two streams, i.e., in-phase and quadrature. Theinformation bits are split into two parts 204, 206 as the inputs of thetwo streams. For example, bits at even positions can be provided to onestream and bits at odd positions can be provided to the other stream.

Each stream performs pulse-amplitude modulation (PAM) to the incominginformation bits. An in-phase PAM module 208 performs in-phase PAM forthe first stream of information bits 204. A quadrature PAM module 210performs quadrature PAM for the second stream of information bits 206.For M-QAM, the real and imaginary parts are modulated by √{square rootover (M)}-PAM, i.e.,Re{X[m]}, Im{X[m]}∈{±A, ±3A, . . . , ±(√{square root over (M)}−1)A},where A is a constant used to normalize the average symbol power, i.e.,

${\frac{1}{\sqrt{M}}{\sum\limits_{k = 1}^{\frac{\sqrt{M}}{2}}\;{2\left( {{2\; k} - 1} \right)^{2}A^{2}}}} = 1$For example, for 4 QAM (QPSK),

${A = {{\frac{1}{2}.\mspace{14mu}{For}}\mspace{14mu} 16\;{QAM}}},{A = {\frac{1}{\sqrt{10}}.}}$

Gray code can be used in the modulation mapping to minimize the biterror rate (BER). For example, for 4 PAM, the bit mapping to symbols{−3A, −A, A, 3A} is {00,01,11,10}. For 8 PAM, the bit mapping to symbols{−7A, −5A, −3A, −A, A, 3A, 5A, 7A} is {000,001,011,010,110,111,101,100}.FIGS. 9 and 10 show the Gray-coded bit/symbol mapping for 4 QAM (QPSK)and 16 QAM.

At an up-sampling module 212, the first modulated symbol sequence isup-sampled by a back-off rate D to reduce the intersymbol interferencebut sacrificing the information rate, i.e., decreasing the averagenumber of information bits transmitted per time slot. Similarly, at anup-sampling module 214, the second modulated symbol sequence isup-sampled by a back-off rate D. The up-sampling can be expressed asfollows. The input symbols X[m] are fed into the up-sampler modules 212or 214, and the output symbols S[m] can be expressed as

${S\lbrack m\rbrack} = \left\{ {\begin{matrix}{{X\left\lbrack \frac{m}{D} \right\rbrack},} & {{{{if}\mspace{14mu} m} = {kD}},{k = 0},1,2,\ldots} \\{0,} & {otherwise}\end{matrix}.} \right.$

The complex symbols S[m]=S_(I)[m]+jS_(Q)[m] are then convolved with thecomplex signature waveform W[m]=W_(I)[m]+jW_(Q)[m] as shown in FIG. 7.An in-phase signature waveform convolution module 216 calculates theconvolution of the in-phase up-sampled symbols S_(I) 218 and thein-phase portion of the signature waveform W_(I). A quadrature signaturewaveform convolution module 220 calculates the convolution of thein-phase up-sampled symbols S_(I) 218 and the quadrature portion of thesignature waveform W_(Q). An in-phase signature waveform convolutionmodule 222 calculates the convolution of the quadrature up-sampledsymbols S_(Q) 224 and the in-phase portion of the signature waveformW_(I). A quadrature signature waveform convolution module 226 calculatesthe convolution of the quadrature up-sampled symbols S_(I) 224 and thequadrature portion of the signature waveform W_(Q). The convolution canbe expressed asS[m]*W[m]=(S _(I) [m]*W _(I) [m]−S _(Q) [m]*W _(Q) [m])+j(S_(I) [m]*W_(Q) [m]+S _(Q) [m]*W _(I) [m]).

An adder 228 adds an output 230 of the in-phase signature waveformconvolution module 216 and an output 232 of the quadrature signaturewaveform convolution module 226 to generate the in-phase portion 234 ofthe convolution S[m]*W[m]. An adder 236 adds an output 238 of thequadrature signature waveform convolution module 220 and an output 240of the in-phase signature waveform convolution module 222 to generatethe in-phase portion 242 of the convolution S[m]*W[m].

The in-phase portion 234 of the convolution is passed through adigital-to-analog converter (D/A) 244 and is modulated by an in-phasecarrier signal cos 2πf_(c)t at a mixer 246 to generate an in-phaseportion 248 of a downlink transmission signal 258. Similarly, thequadrature portion 242 of the convolution is passed through adigital-to-analog converter 250 and is modulated by a quadrature carriersignal sin 2πf_(c)t at a mixer 252 to generate a quadrature portion 254of the downlink transmission signal 258. An adder 256 adds the in-phaseportion 248 and quadrature portion 254 to generate the downlinktransmission signal 258.

Referring to FIG. 8, in some implementations, a 260 modulates a firststream of information bits 262 and a second stream of information bits264, and processes the two streams of modulated symbols to generate acombined downlink signal 266. The first stream of information bits 262can be intended for a first terminal device, and the second stream ofinformation bits 264 can be intended for a second terminal device. Theinformation bits 262 are modulated into complex symbols using quadratureamplitude modulation at an in-phase PAM module 268 and a quadrature PAMmodule 270. The in-phase and quadrature PAM modulated symbols areup-sampled at the up-sampling modules 272 and 274, respectively. Thein-phase up-sampled symbols are convolved with the signature waveform atthe in-phase signature waveform convolution module 276 and quadraturesignature waveform convolution module 278. The quadrature up-sampledsymbols are convolved with the signature waveform at the in-phasesignature waveform convolution module 280 and quadrature signaturewaveform convolution module 282. An adder 284 adds the output of thein-phase signature waveform convolution module 276 and the output of thequadrature signature waveform convolution module 282 to generate thein-phase portion 312 of the convolution S₁[m]*W[m] for the first streamof symbols. An adder 286 adds the output of the quadrature signaturewaveform convolution module 278 and the output of the in-phase signaturewaveform convolution module 280 to generate the quadrature portion 316of the convolution S₁[m]*W[m] for the first stream of symbols.

The second stream of information bits 264 are modulated into complexsymbols using quadrature amplitude modulation at an in-phase PAM module288 and a quadrature PAM module 290. The in-phase and quadrature PAMmodulated symbols are up-sampled at the up-sampling modules 292 and 294,respectively. The in-phase up-sampled symbols are convolved with thesignature waveform at the in-phase signature waveform convolution module296 and quadrature signature waveform convolution module 298. Thequadrature up-sampled symbols are convolved with the signature waveformat the in-phase signature waveform convolution module 300 and quadraturesignature waveform convolution module 302. An adder 304 adds the outputof the in-phase signature waveform convolution module 296 and the outputof the quadrature signature waveform convolution module 302 to generatethe in-phase portion 314 of the convolution S₂[m]*W[m] for the secondstream of symbols. An adder 306 adds the output of the quadraturesignature waveform convolution module 298 and the output of the in-phasesignature waveform convolution module 300 to generate the quadratureportion 318 of the convolution S₂[m]*W[m] for the second stream ofsymbols.

An adder 308 adds the in-phase portion 312 of the convolution S₁[m]*W[m]for the first stream of symbols and the in-phase portion 314 of theconvolution S₂[m]*W[m] for the second stream of symbols to generate acombined in-phase signal 320. The combined in-phase signal 320 is passedthrough a digital-to-analog converter 244 and modulated by an in-phasecarrier signal cos 2πf_(c)t at a mixer 328 to generate an in-phaseportion 332 of a combined downlink transmission signal 336. An adder 310adds the quadrature portion 316 of the convolution S₁[m]*W[m] for thefirst stream of symbols and the quadrature portion 318 of theconvolution S₂[m]*W[m] for the second stream of symbols to generate acombined quadrature signal 322. The combined quadrature signal 322 ispassed through a digital-to-analog converter 326 and modulated by aquadrature carrier signal sin 2πf_(c)t at a mixer 330 to generate aquadrature portion 334 of the downlink transmission signal 336. An adder338 adds the in-phase portion 332 and quadrature portion 334 to generatethe combined downlink transmission signal 336.

The example of FIG. 8 shows a transmitter that combines two streams ofinformation bits intended for two terminal devices. The same principlecan be applied to processing additional streams of information bitsintended for additional terminal devices. An adder adds the in-phaseportion of the convolution S_(i)[m]*W[m] for each of the streams ofsymbols to generate a combined in-phase signal that is passed through adigital-to-analog converter and modulated by an in-phase carrier signalcos 2πf_(c)t to generate an in-phase portion of a combined downlinktransmission signal. An adder adds the quadrature portion of theconvolution S_(i)[m]*W[m] for each of the streams of symbols to generatea combined quadrature signal that is passed through a digital-to-analogconverter and modulated by a quadrature carrier signal sin 2πf_(c)t togenerate a quadrature portion of the downlink transmission signal. Anadder adds the in-phase portion and the quadrature portion to generatethe combined downlink transmission signal.

A feature of the time-reversal system 150 is that after the informationbits are pulse amplitude modulated and convolved with the signaturewaveforms, the signals corresponding to different streams of informationbits can be carrier modulated using the same carrier frequency, addedtogether, and transmitted through an antenna at the same time.

Other approaches such as the polar form method can be applied toimplement the transmitter 200 of FIG. 7 or 260 of FIG. 8. The polar formmethod is to implement the multiplication of two complex values by firstmultiplying the amplitudes of the two complex values, adding the phasesof the two complex values, and then combining the multiplied amplitudeand phase into one complex value. In polar form multiplication, if twocomplex values are to be multiplied, say p=|p|e^(j)*^(arg(p)) andq=|q|e^(j)*^(arg(q)) where arg(·) denotes the phase, then themultiplication can be obtained by p·q=(|p|·|q|)e^(j(arg(p)+arg(q))).

FIG. 11 is a block diagram of an example receiver 340 in the quadratureamplitude modulation time-reversal system 150. For example, the receiver340 can be implemented in the terminal device 154 for receiving downlinksignals transmitted by the base station 152. A received waveform 342(e.g., transmitted by one of the terminal devices) is divided into afirst stream 344 and a second stream 346. The first stream 344 isdemodulated at a mixer 348 by an in-phase carrier signal cos 2πf_(c)t togenerate a demodulated signal 352 that is passed through ananalog-to-digital converter 356. The second stream 346 is demodulated ata mixer 350 by a quadrature carrier signal sin 2πf_(c)t to generate ademodulated signal 354 that is passed through an analog-to-digitalconverter 358. An output 360 of the analog-to-digital converter 356 isdown-sampled at a down-sampling module 366 to generate a down-sampledin-phase stream 370. An output 364 of the analog-to-digital converter358 is down-sampled at a down-sampling module 368 to generate adown-sampled quadrature stream 372.

A symbol timing synchronization module 362 generates a timingsynchronization signal provided to the down-sampling modules 366 and 368so that the down-sampling modules 366 and 268 sample the signals withcorrect timing. The down-sampled in-phase stream 370 is demodulatedusing PAM demodulation, which is a symbol/bit mapping, to generate ademodulated in-phase stream 378. The down-sampled quadrature stream 372is demodulated using PAM demodulation to generate a demodulatedquadrature stream 380. The demodulated in-phase stream 378 anddemodulated quadrature stream 380 are concatenated at a concatenationmodule 382 to generate the estimated bits 384 of the originaltransmitted bits. If the receiver 340 demodulates a downlink signaltransmitted by the transmitter 200 of FIG. 7, the estimated bits 384 inFIG. 11 correspond to the information bits 202 of FIG. 7. If there areno errors during transmission, the estimated bits 384 will be the sameas the information bits 202.

Error! Reference source not found. below lists the achievable bit ratesusing various orders of quadrature amplitude modulations and variousrate back-off factors. A higher order modulation and lower rate back-offfactor can provide a higher bit rate.

TABLE 1 Bit rates with different rate back-off factors and QAMs BPSKQPSK 16QAM 64QAM 256QAM D = 1  1 Gbps  2 Gbps  4 Gbps 6 Gbps 8 Gbps D =2 500 Mbps  1 Gbps  2 Gbps 3 Gbps 4 Gbps D = 4 250 Mbps 500 Mbps  1 Gbps1.5 Gbps   2 Gbps D = 8 125 Mbps 250 Mbps 500 Mbps 750 Mbps  1 Gbps

The following describes simulation results for quadrature amplitudemodulation in a wireless time-reversal system. Referring to FIG. 12,graphs 390 and 392 show example in-phase and quadrature portions of achannel impulse response.

Referring to FIG. 13, graphs 390, 392, 394, and 396 show in-phase QPSKmodulated waveforms for the symbols 00, 01, 10, and 11, respectively(before the digital-to-analog converter, e.g., 244 of FIG. 7). Graphs398, 400, 402, and 404 show quadrature QPSK modulated waveforms for thesymbols 00, 01, 10, and 11, respectively (before the digital-to-analogconverter, e.g., converter 250 of FIG. 7).

Referring to FIG. 14, graphs 410, 412, 414, and 416 show the in-phaseportions of the received signals for each of the transmitted symbols 00,01, 10, and 11, respectively (after the analog-to-digital converter,e.g., 356 of FIG. 11). Graphs 418, 420, 422, and 424 show the quadratureportions of the received signals for each of the transmitted symbols 00,01, 10, and 11, respectively (after the analog-to-digital converter,e.g., 358 of FIG. 11). The receiver determines the received signal basedon a particular sample, e.g., a peak (e.g., 426) in the received signalby applying PAM demodulation on the particular sample to generatedemodulated symbols. The polarity of the peaks (e.g., 426, 428, 430,432) of the in-phase and quadrature received signals correspond to thereal and imaginary parts of the original transmitted symbol.

Referring to FIG. 15, a graph 450 shows exemplary numerical simulationresults for bit error rate (BER) performance of a time-reversal systemat 1 Giga bit per second (Gbps). The sampling frequency is 1 Giga Hz(GHz). The legend “TR” represents the time-reversal system that uses thebasic time reversal waveform. The legend “MSINR” represents thetime-reversal system that modifies the waveforms to maximizesignal-to-interference-and-noise-ratio. For the same bit rate, using asmaller rate back-off factor D achieves a better performance. A smallerrate back-off factor D allows the constellation size to be smaller,i.e., the minimum distance between symbols can be larger. However, asmaller rate back-off factor D may result in larger inter-symbolinterference that degrades the signal-to-interference-and-noise ratio.The benefit of increased minimum distance overrides thesignal-to-interference-and-noise ratio degradation and thus provides abetter bit error rate performance.

Referring to FIG. 16, a graph 460 shows a comparison of the bit errorrate performance when the rate back-off factor D is fixed as 8 and theconstellation sizes are varied.

FIG. 17 is a flow diagram of a process 470 for time-reversal wirelesscommunication using quadrature amplitude modulation. For example, theprocess 470 can be implemented by the transmitter 200 (FIG. 7) or 260(FIG. 8). The process 470 includes at a base station, receiving a probesignal from a terminal device 472. A signature waveform is generatedbased on a time-reversed signal of a channel response signal derivedfrom the probe signal 474. Quadrature amplitude modulation is performedon a transmit signal to generate a quadrature amplitude modulated signal476. A transmission signal is generated based on the quadratureamplitude modulated signal and the signature waveform 478. FIG. 18 is aflow diagram of an example process 480 for time-reversal wirelesscommunication using quadrature amplitude modulation. For example, theprocess 480 can be implemented by the transmitter 200 (FIG. 7) or 260(FIG. 8). The process 480 includes performing quadrature amplitudemodulation on a first transmit signal to generate a first quadratureamplitude modulated signal 482. Quadrature amplitude modulation isperformed on a second transmit signal to generate a second quadratureamplitude modulated signal 484. A first transmission signal is generatedbased on the first quadrature amplitude modulated signal and a firstsignature waveform associated with a first terminal device 486. A secondtransmission signal is generated based on the second quadratureamplitude modulated signal and a second signature waveform associatedwith a second terminal device 488. A combined transmission signal isgenerated by adding a real part of the first transmission signal with areal part of the second transmission signal to generate an in-phase partof the combined transmission signal, and adding an imaginary part of thefirst transmission signal with an imaginary part of the secondtransmission signal to generate a quadrature part of the combinedtransmission signal 490.

In some implementations, the waveforms sent from the base station 152 tothe terminals 154 can be designed to optimize sum rate by suppressinginter-symbol interference and inter-user interference, as described inU.S. patent application Ser. No. 13/706,342, filed on Dec. 5, 2012,titled “Waveform Design for Time-Reversal Systems,” herein incorporatedby reference in its entirety. For example, the base station 152 mayreceive channel response signals derived from probe signals sent fromterminal devices 154, in which each probe signal is sent from one of theterminal devices 154 to the base station 152 through multiplepropagation paths. The probe signals may be sent during the channelprobing phase 170 (FIG. 5). The base station 152 may determine downlinkwaveforms for the terminal devices 154 to increase a weighted sum-rateunder a total power constraint, in which the downlink waveforms aredetermined based on time-reversed channel response signals and initialvirtual uplink power allocation coefficients. The base station 152 maydetermine updated virtual uplink power allocation coefficients based onthe downlink waveforms, and determine downlink power allocationcoefficients based on the downlink waveforms and the virtual uplinkpower allocation coefficients.

The base station 152 may determine virtual uplinksignal-to-interference-and-noise ratios (SINRs) based on the virtualuplink power allocation coefficients, and determine the downlink powerallocation coefficients based on the virtual uplink SINRs. The virtualuplink SINRs may take into account both inter-symbol interference (ISI)and inter-user interference (IUI). Increasing the weighted sum-rate mayalso reduce a combination of the inter-symbol interference and theinter-user interference. The base station 152 may transmit a downlinksignal derived from a combination of the downlink waveforms and takeinto account the downlink power allocation coefficients, in which thedownlink signal is transmitted to each of the terminal devices 154through multiple propagation paths. The downlink signal can beconfigured to enable each terminal device 154 to receive multipathsignals that can be used determine a data signal intended for theterminal device 154, in which different terminal devices 154 receive thedownlink signal through different propagation paths and determinedifferent data signals, and in which the downlink signal is configuredto reduce inter-symbol interference (ISI) and inter-user interference(IUI) for the data signals determined at the terminal devices 154. Thebase station 152 may determine the downlink waveforms and the virtualuplink power allocation coefficients by iteratively determining updateddownlink waveforms based on previously determined virtual uplink powerallocation coefficients, and determining updated virtual uplink powerallocation coefficients based on previously determined downlinkwaveforms. The base station 152 may determine downlink waveforms bydetermining downlink waveforms to maximize the weighted sum-rate underthe total power constraint. The base station 152 may determine downlinkwaveforms by determining minimum mean squared error (MMSE) waveforms.

In some implementations, the complexities of the base station 152 andthe terminal devices 154 are asymmetric, such that the base station 152performs most of the signal processing as both a transmitter (for thedownlink) and receiver (for the uplink), allowing the use of lowcomplexity terminal devices 154, as described in U.S. patent applicationSer. No. 13/969,271, filed on Aug. 16, 2013, titled “Time-ReversalWireless Systems Having Asymmetric Architecture,” herein incorporated byreference in its entirety.

In some implementations, the multi-user time reversal communicationsystem 150 can use a two-dimensional parallel interference cancellationscheme to enhance the system performance, as described in U.S. patentapplication Ser. No. 13/969,320, filed on Aug. 16, 2013, titled“Multiuser Time-Reversal Divisional Multiple Access Uplink System withParallel Interference Cancellation,” herein incorporated by reference inits entirety. The two-dimensional parallel interference cancellationscheme uses tentative decisions of detected symbols to effectivelycancel the interference in both the time dimension (inter-symbolinterference) and the user dimension (inter-user interference), whichsignificantly improves the bit-error-rate performance to achieve a highsignal-to-noise-ratio. To further improve the bit error rateperformance, a multi-stage processing can be implemented by cascadingmultiple stages of the proposed two-dimensional interferencecancellation, with a total delay that increases linearly with the numberof stages, but independent of the number of users.

In some implementations, the base station 152 can be part of a mobile orstationary device. For example, the base station 152 can be implementedas part of a sensor module, a controller, a mobile phone, a laptopcomputer, or an electronic appliance that communicates wirelessly withmultiple other devices. For example, a mobile phone or a laptop computermay communicate simultaneously with a television, a printer, athermometer, a radio, a refrigerator, a lighting control system, andother devices using the techniques described above.

The wireless time-reversal system 150 can be used to implement theInternet of Things, as described in U.S. patent application Ser. No.14/202,651, filed on Mar. 10, 2014, titled “Time-Reversal WirelessParadigm For Internet of Things,” herein incorporated by reference inits entirety.

The base station 152 can include one or more processors and one or morecomputer-readable mediums (e.g., RAM, ROM, SDRAM, hard disk, opticaldisk, and flash memory). The one or more processors can perform variouscalculations described above. The calculations can also be implementedusing application-specific integrated circuits (ASICs). The term“computer-readable medium” refers to a medium that participates inproviding instructions to a processor for execution, including withoutlimitation, non-volatile media (e.g., optical or magnetic disks), andvolatile media (e.g., memory) and transmission media. Transmission mediaincludes, without limitation, coaxial cables, copper wire and fiberoptics.

The features described above can be implemented advantageously in one ormore computer programs that are executable on a programmable systemincluding at least one programmable processor coupled to receive dataand instructions from, and to transmit data and instructions to, a datastorage system, at least one input device, and at least one outputdevice. A computer program is a set of instructions that can be used,directly or indirectly, in a computer to perform a certain activity orbring about a certain result. A computer program can be written in anyform of programming language (e.g., C, Java), including compiled orinterpreted languages, and it can be deployed in any form, including asa stand-alone program or as a module, component, subroutine, abrowser-based web application, or other unit suitable for use in acomputing environment.

Suitable processors for the execution of a program of instructionsinclude, e.g., both general and special purpose microprocessors, digitalsignal processors, and the sole processor or one of multiple processorsor cores, of any kind of computer. Generally, a processor will receiveinstructions and data from a read-only memory or a random access memoryor both. The essential elements of a computer are a processor forexecuting instructions and one or more memories for storing instructionsand data. Generally, a computer will also include, or be operativelycoupled to communicate with, one or more mass storage devices forstoring data files; such devices include magnetic disks, such asinternal hard disks and removable disks; magneto-optical disks; andoptical disks. Storage devices suitable for tangibly embodying computerprogram instructions and data include all forms of non-volatile memory,including by way of example semiconductor memory devices, such as EPROM,EEPROM, and flash memory devices; magnetic disks such as internal harddisks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROMdisks. The processor and the memory can be supplemented by, orincorporated in, ASICs (application-specific integrated circuits).

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

A number of implementations of the invention have been described.Nevertheless, it will be understood that various modifications can bemade without departing from the spirit and scope of the invention. Forexample, the transmitters 200, 260, and the receiver 340 can includemore components that are not shown in the figures. The quadratureamplitude modulation can be, e.g., 4-QAM, 16-QAM, 64-QAM, or 256-QAM.The method of transforming information bits into symbols can bedifferent from what is described above. In the example of FIG. 7, theinformation bits 202 are separated into a two streams, in which bits ateven positions are provided to one stream and bits at odd positions areprovided to the other stream. The bits can be separated into two streamsusing other methods. In this description, quadrature phase shift keying(QPSK) is considered one type of quadrature amplitude modulation, i.e.,4QAM. The time-reversal system 150 can also transmit analog signals. Thesignature waveform can be a modified version of the time-reversedchannel response signal. For example, the signature waveform can bedesigned to reduce interference. To generate the signature waveform, itis not necessary to first generate a time-reversed signal of the channelresponse signal. For example, the time-reverse operation may be movedtoward a later stage of the process, i.e., design the signature based onthe channel response and then time-reverse the resulting signature.Another method of generating the signature waveform is to separate thetime-reverse operation into many element-swap operations and distributethese operations in the signature waveform design algorithm.

Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A method comprising: at a base station, receivinga first probe signal from a first terminal device; generating a firstsignature waveform that is based on a first channel response signalderived from the first probe signal; at the base station, receiving asecond probe signal from a second terminal device; generating a secondsignature waveform that is based on a second channel response signalderived from the second probe signal; jointly determining modified firstand second signature waveforms to increase a sum rate by suppressinginter-symbol-interference and inter-user-interference; performingquadrature amplitude modulation (QAM) on a first transmit signal togenerate a first quadrature amplitude modulated signal, the firsttransmit signal intended to be received by the first terminal device;performing quadrature amplitude modulation (QAM) on a second transmitsignal to generate a second quadrature amplitude modulated signal, thesecond transmit signal intended to be received by the second terminaldevice; generating a first transmission signal based on the firstquadrature amplitude modulated signal and the first modified signaturewaveform; generating a second transmission signal based on the secondquadrature amplitude modulated signal and the second modified signaturewaveform; and generating a combined transmission signal based on thefirst and second transmission signals.
 2. The method of claim 1 in whichperforming quadrature amplitude modulation on the first transmit signalcomprises: dividing the first transmit signal into a first part and asecond part, applying amplitude modulation on the first part to generatean in-phase part of the first quadrature amplitude modulated signal, andapplying amplitude modulation on the second part to generate aquadrature part of the first quadrature amplitude modulated signal. 3.The method of claim 1 in which the first signature waveform comprises acomplex signal having a real part and an imaginary part.
 4. The methodof claim 1 in which generating a first signature waveform comprisesgenerating a first signature waveform that is based on a time-reversedsignal of the first channel response signal.
 5. The method of claim 1 inwhich generating the first transmission signal comprises performing aconvolution of the first signature waveform and the first quadratureamplitude modulated signal or a modified version of the first quadratureamplitude modulated signal.
 6. The method of claim 5 in which themodified version of the first quadrature amplitude modulated signalcomprises an up-sampled version of the first quadrature amplitudemodulated signal, and generating the transmission signal comprisesperforming a convolution of the first signature waveform and theup-sampled version of the first quadrature amplitude modulated signal.7. The method of claim 1 in which the first transmit signal comprises afirst digital transmit signal.
 8. The method of claim 7 in whichperforming the first quadrature amplitude modulation on the firsttransmit signal comprises encoding data bits of the first transmitsignal based on Gray codes, and mapping Gray-coded data bits toquadrature amplitude modulated symbols.
 9. The method of claim 1 inwhich generating the first signature waveform comprises generating afirst signature waveform that is based on a time-reversed conjugatesignal of the first channel response signal modified to increase the sumrate by suppressing inter-symbol-interference andinter-user-interference.
 10. The method of claim 1 in which performingquadrature amplitude modulation on a first transmit signal comprisesperforming at least one of 4QAM, 16 QAM, 64 QAM, or 256 QAM.
 11. Anapparatus comprising: a first device comprising: an input circuitconfigured to receive a first probe signal transmitted wirelessly from asecond device through a first set of multiple propagation paths, andreceive a second probe signal transmitted wirelessly from a third devicethrough a second set of multiple propagation paths; and a data processorconfigured to: generate a first signature waveform that is based on afirst channel response signal derived from the first probe signal;generate a second signature waveform that is based on a second channelresponse signal derived from the second probe signal; and jointlydetermine modified first and second signature waveforms to increase asum rate by suppressing inter-symbol-interference andinter-user-interference; perform a first quadrature amplitude modulation(QAM) on a first transmit signal to generate a first quadratureamplitude modulated signal; perform a second quadrature amplitudemodulation (QAM) on a second transmit signal to generate a secondquadrature amplitude modulated signal; generate a first transmissionsignal based on the first quadrature amplitude modulated signal and thefirst modified signature waveform; generate a second transmission signalbased on the second quadrature amplitude modulated signal and the secondmodified signature waveform; and generate a combined transmission signalbased on the first and second transmission signals.
 12. The apparatus ofclaim 11 in which perform quadrature amplitude modulation on the firsttransmit signal comprises: divide the first transmit signal into a firstpart and a second part, apply amplitude modulation on the first part togenerate an in-phase part of the first quadrature amplitude modulatedsignal, and apply amplitude modulation on the second part to generate aquadrature part of the first quadrature amplitude modulated signal. 13.The apparatus of claim 11 in which the first signature waveformcomprises a complex signal having a real part and an imaginary part. 14.The apparatus of claim 11 in which the data processor is configured togenerate the first signature waveform based on a time-reversed signal ofthe channel response signal.
 15. The apparatus of claim 11 in whichgenerate the first transmission signal comprises perform a convolutionof the first signature waveform and the first quadrature amplitudemodulated signal or a modified version of the first quadrature amplitudemodulated signal.
 16. The apparatus of claim 15 in which the modifiedversion of the first quadrature amplitude modulated signal comprises anup-sampled version of the first quadrature amplitude modulated signal,and generate the first transmission signal comprises perform aconvolution of the first signature waveform and the up-sampled versionof the first quadrature amplitude modulated signal.
 17. The apparatus ofclaim 11 in which the first transmit signal comprises a first digitaltransmit signal.
 18. The apparatus of claim 17 in which perform thequadrature amplitude modulation on the first transmit signal comprisesencode data bits of the first transmit signal based on Gray codes, andmap Gray-coded data bits to quadrature amplitude modulated symbols. 19.The apparatus of claim 11 in which generate the first signature waveformcomprises generate a first signature waveform that is based on atime-reversed conjugate signal of the first channel response signalmodified to increase the sum rate by suppressinginter-symbol-interference and inter-user-interference.
 20. The apparatusof claim 11 in which perform quadrature amplitude modulation on a firsttransmit signal comprises perform at least one of 4 QAM, 16 QAM, 64 QAM,or 256 QAM.
 21. The apparatus of claim 11, further comprising a mobilephone, a car, a boat, an airplane, a robot, an unmanned aerial vehicle,a thermostat, a refrigerator, a lighting control system, a tabletcomputer, a network router, or a television that includes the firstdevice.
 22. The apparatus of claim 11 in which perform quadratureamplitude modulation on the first transmit signal comprises: divide thefirst transmit signal into a first part and a second part, apply anin-phase pulse amplitude modulation (PAM) on the first part to generatean in-phase part of the first quadrature amplitude modulated signal, andapply a quadrature PAM on the second part to generate a quadrature partof the first quadrature amplitude modulated signal.
 23. The apparatus ofclaim 22 in which generate the first transmission signal comprises:convolve the in-phase part of the first quadrature amplitude modulatedsignal with an in-phase part of the first signature waveform to generatea first signal; convolve the in-phase part of the first quadratureamplitude modulated signal with a quadrature part of the first signaturewaveform to generate a second signal; convolve the quadrature part ofthe first quadrature amplitude modulated signal with the in-phase partof the first signature waveform to generate a third signal; convolve thequadrature part of the first quadrature amplitude modulated signal withthe quadrature part of the first signature waveform to generate a fourthsignal; add the first signal and the fourth signal to generate a fifthsignal; and add the second signal and the third signal to generate a sixsignal.
 24. The apparatus of claim 23 in which perform quadratureamplitude modulation on the second transmit signal comprises: divide thesecond transmit signal into a third part and a fourth part, applyin-phase PAM on the third part to generate an in-phase part of thesecond quadrature amplitude modulated signal, and apply quadrature PAMon the fourth part to generate a quadrature part of the secondquadrature amplitude modulated signal.
 25. The apparatus of claim 24 inwhich generate the second transmission signal comprises: convolve thein-phase part of the second quadrature amplitude modulated signal withan in-phase part of the second signature waveform to generate a seventhsignal; convolve the in-phase part of the second quadrature amplitudemodulated signal with a quadrature part of the second signature waveformto generate an eighth signal; convolve the quadrature part of the secondquadrature amplitude modulated signal with the in-phase part of thesecond signature waveform to generate a ninth signal; convolve thequadrature part of the second quadrature amplitude modulated signal withthe quadrature part of the second signature waveform to generate a tenthsignal; add the seventh signal and the tenth signal to generate aneleventh signal; and add the eighth signal and the ninth signal togenerate a twelfth signal.
 26. The apparatus of claim 25 in whichgenerate the combined transmission signal comprises: add the fifthsignal and the eleventh signal to generate a thirteenth signal; and addthe sixth signal and the twelfth signal to generate a fourteenth signal.